core.logic goal failure check

fail-goal.clj

; clojure 1.4.0, core.logic 0.8.0-beta2

user=> (use 'clojure.core.logic)
nil
user=> (defn list?o
        [v]
        (conde
          [(emptyo v)]
          [(fresh [f] (firsto v f))]))
#'user/list?o

user=> (run* [q] (list?o [1 2 3]))
(_.0)

user=> (run* [q] (list?o 'x))
()

user=> (defn fail-goal [g]
          (fn [a]
            (if (nil? (g a))
              a)))
#'user/fail-goal

user=> (run* [q] (fail-goal succeed))
()

user=> (run* [q] (fail-goal fail))
(_.0)

; looks good

user=> (run* [q] (fail-goal (list?o [1 2 3])))
()

; still good

user=> (run* [q] (fail-goal (list?o 'x)))
()
; wait, what?